Non-ribbing ferritic steel and process

ABSTRACT

Low carbon ferritic stainless steel containing about 14-18 percent chromium, 0.0005 to 0.005 percent boron, up to about 0.12 percent, and preferably 0.02 to 0.12 percent columbium, characterized by substantial freedom from ribbing in unidirectionally hot rolling and thence unidirectionally cold rolling into thin gauge strip; also cold rolled strip in long lengths made of said steel and methods of producing same.

United States Patent [191 Fogarty et al.

NON-RIBBING FERRITIC STEEL AND PROCESS Inventors: John Edward Fogarty; Stanley Paul Odar, both of Canton; Glenn Donald Angerman, Massillon, all of Ohio Republic Steel Corporation, Cleveland, Ohio Filed: Oct. 15, 1971 Appl. No.: 189,787

Related US. Application Data Division of Ser. No. 814,188, July 7, 1969, abandoned.

Assignee:

References Cited UNITED STATES PATENTS 10/1957 Leffingwell et al. 148/12 6/1956 Loveless 75/128 F Aug. 21, 1973 2,905,577 9/1959 Harris et a1 148/31 2,965,479 12/1960 Evans 75/126 3,128,211 4/1964 Waxweiler 148/135 X 3,309,238 3/1967 Randak et al.... 148/135 X 3,490,956 l/l970 Wilton 148/12 3,655,459 4/1972 Brickner... 148/12 3,139,358 9/1964 Graziano 148/12 OTHER PUBLlCATlONS Proceedings of Electric Furnace Conference, 1961, Non-Roping Ferritic Chromium Steels, Thompson et al., pgs. 70-85.

Primary Examiner--Charles N. Lovell Attorney-Raymond J. McElhannon [5 7] ABSTRACT Low carbon ferritic stainless steel containing about 14-18 percent chromium, 0.0005 to 0.005 percent boron, up to about 0.12 percent, and preferably 0.02 to 0.12 percent columbium, characterized by substantial freedom from ribbing in unidirectionally hot rolling and thence unidirectionally cold rolling into thin gauge strip; also cold rolled strip in long lengths made of said steel and methods of producing same.

4 Claims, No Drawings NON-RIBBING FERRITIC STEEL AND PROCESS This application is a division of application Ser. No. 814,818, filed Apr. 7, 1969 (now abandoned).

This invention pertains to improvements in low carbon, ferritic, stainless steel containing about 14 to 18 percent chromium for preventing ribbing in rolling into strip and to the production of unribbed strip therefrom.

Although such steel is produced in large tonnages commercially, such as type 434, in the form of cold rolled strip, it has been found difficult to produce such strip in long lengths or coils free from so called ribbing or ridging.

Ribbing or ridging manifests itself when certain metals or alloys are unidirectionally rolled into strip in long lengths or coils, and occurs when such a product is unequally strained or elongated in the direction of rolling. When strained in this manner, longitudinal ridges occur which are parallel to the rolling direction. Often the ridges are readily apparent to the eye, and the transverse wave-like appearance is especially evident. Obviously, ribbing or ridging is deleterious and cannot be tolerated to any appreciable degree in products manufactured from strip metal. For brevity, this phenomenon will be hereinafter designated as ribbing."

The austenitic stainless steels are, generally speaking, not subject to such ribbing on unidirectionally hot and thence cold rolling into thin strip. But I have discovered that such is not the case in the rolling of the essentially straight chromium, ferritic, stainless steels containing chromium in the range of about 14-18 percent.

We have discovered, in accordance with the present invention, that this ribbing effect occurring in the rolling of chromium stainless steels of the character aforesaid, may be substantially or completely eliminated by incorporating in the steel in the course of melting, a small but significant amount of boron, together preferably with the addition of a small but significant amount of columbium, the boron content constituting about 0.0005 to 0.005 percent by weight of the total, and the columbium content about 0.02-0.l2 percent.

We have further discovered that the aforesaid ribbing tendency is influenced by the size of the ingot from which the strip is produced; the greater the ingot size, the greater the ribbing tendency. We have found, for example, that while this tendency is quite pronounced in strip produced from 25 X 36 and 25 X 41inch ingots, it is not so pronounced and may even not be present with strip produced from ingots of about 20 X 32 inches or less. The apparent reason for this is that ingot thickness influences freezing rate. The thicker ingots freeze more slowly than thinner ingots, thereby producing a coarser and more segregated ingot structure. Boron additions alone or as supplemented by columbium additions within limits above stated, inhibit this natural segregation tendency and thus produce structures more comparable to thinner ingots.

The boron may be alloyed with the essentially straight chromium steel as produced, for example, in the electric furnace by furnace or ladle additions to the molten steel in the form of boron, Fe.boron, borosil, etc. Columbium may be similarly added.

EXAMPLE 1 The ribbing effect and its elimination by boron additions in accordance with the invention, is exemplified in the production and subsequent rolling of type 434 electric furnace steel of base composition: carbon 0.12

percent max., chromium 14-18 percent, molybdenum 0.7-1.0 percent, manganese and silicon 1.0 percent max., each of sulphur and phosphorus 0.030 and 0.040 max., respectively, balance iron except for impurities within commercial tolerances.

A number of heats of this steel were produced and poured into ingots of 20 X 32, 25 X 36 and 25 X 41 inches, respectively. To certain heats, boron additions were made in amounts verified on check analyses to fall within limits of 00006-00044 percent. The ingots were then rolled into automotive body stock of 0.015 to 0.025 inch gauge as follows:

The ingots were heated in soaking pits and hot rolled thence in the blooming mill into 6 A X 18 to 37 inch slabs, one slab per ingot. The slabs were skin ground, reheated, and fed into a 56 inch continuous hot strip mill and hot rolled in a single pass into 0.117 to 0.140 inch X 18 inches to 37 inch coils. The coils were box annealed, then subjected to a continuous blast and pickling in sulphuric, and nitric-hydrofluoric acids. The coils were then cold rolled in a cold reversing mill to 005-006 inch gauge on 60 and grit rolls. The cold rolled coils were then strand annealed, and thence electrolytically pickled in nitric or sulphuric acid. The coils were then further cold rolled in a reversing or Sendzimir mill to finish gauge of 0.015-0.025 inch on 280 and 400 grit rolls. The coils were then atmosphere annealed in an atmosphere of cracked ammonia gas. The coils were then given a skin or temper pass. The product was rated for ribbing at finish gauge following the temper pass operation.

It was found that ribbing was completely eliminated from all stock containing boron additions within the above limits, whereas ribbing was present in all stock containing no boron addition and was particularly severe in the product rolled from the 25 X 36 and 25 X 41 inch ingots.

EXAMPLE III In order to demonstratethe effects of both boron plus columbium additions, an electric furnace heat was produced of modified type 434 steel containing 12 points of columbium, this heat being of the following analysis: 0.077% C, 0.36% Mn, 0.019% P, 0.012% S, 0.05% Si, 0.10% Cu, 0.26% Ni, 16.70% Cr, 0.84% Mo, 0.008% Sn, 0.020% A1, 0.12% (lb, 0.001% Pb. This heat was treated in the ladle with borosil to contain 0.0015-0.002 percent boron.

The heat was cast into ingots and processed as above described in Example I into hot rolled and thence into cold rolled strip in coils, some in strip widths of 27 A inches and others in strip widths of 19 A inches. The narrow band strip showed no ribbing at any time during the processing. The wide band strip showed light ribbing at intermediate and finish gauge anneals which, however, was substantially eliminated by the final skinning or temper pass.

The tests established that columbium plus boron additions completely eliminated ribbing in the narrow band stock and reduced the sarne within acceptable levels in the wider band stock.

What is claimed is:

l. The method of producing cold rolled strip in long lengths substantially free from riblbing, from a ferritic, stainless steel containing about 14- to 18 percent chromium, balance substantially iron, which comprises: producing a molten bath of said steel, alloying therestrip is cold rolled to final gauge of about 0.015 to 0.025 inch.

4. The method according to claim 1 wherein said steel as cast consists essentially of about: l4 to 18 percent chromium, 0.7 to 1 percent molybdenum, 0.0005 to 0.005 percent boron, 0.02 to 0.12% columbium, up to 0.12% carbon, up to 1% each of manganese and silicon, up to 0.03 and 0.04% sulphur and phosphorus, respectively, balance iron, except for impurities within commercial tolerances. 

2. The method according to claim 1 wherein said steel is cast into ingots of at least 25 X 36 inches in size.
 3. The method according to claim 2 wherein said strip is cold rolled to final gauge of about 0.015 to 0.025 inch.
 4. The method according to claim 1 wherein said steel as cast consists essentially of about: 14 to 18 percent chromium, 0.7 to 1 percent molybdenum, 0.0005 to 0.005 percent boron, 0.02 to 0.12% columbium, up to 0.12% carbon, up to 1% each of manganese and silicon, up to 0.03 and 0.04% sulphur and phosphorus, respectively, balance iron, except for impurities within commercial tolerances. 